are that was programmed before a 2005 law decreed that daylight-saving time would start three weeks earlier and end one week later, beginning this year. Congress decided that more early evening daylight would translate into energy savings.

Software created earlier is set to automatically advance its timekeeping by one hour on the first Sunday in April, not the second Sunday in March (that's March 11 this year).

The result is a glitch reminiscent of the Y2K bug, when cataclysmic crashes were feared if computers interpreted the year 2000 as 1900 and couldn't reconcile time appearing to move backward. This bug is much less threatening, but it could cause head-scratching episodes when some computers are an hour off.

The problem won't show up only in computers, of course. It will affect plenty of non-networked devices that store the time and automatically adjust for daylight saving, like some digital watches and clocks. But in those instances the result will be a nuisance (adjust the time manually or wait three weeks) rather than something that might throw a wrench in the works.

Cameron Haight, a Gartner Inc. analyst who has studied the potential effects of the daylight-saving bug, said it might force transactions occurring within one hour of midnight to be recorded on the wrong day. Computers might serve up erroneous information about multinational teleconference times and physical-world appointments.

"Organizations could face significant losses if they are not prepared," the Information Technology Association of America cautioned this week.

Dave Thewlis, who directs CalConnect, a consortium that develops technology standards for calendar and scheduling software, said it is hard to know how widespread the problem will be.

That's because the world is full of computer systems that have particular methods for accounting for time of day. In many, changing the rules around daylight saving is a snap, but in others, it may be more complex.

"There's no rule that says you have to represent time in a certain way if you write a program," Thewlis said. "How complicated it is to implement the change has to do with the original design, where code is located."

Further confounding matters, there are lots of old computer programs whose original vendors don't support them anymore, meaning there's no repair available. Some gadgets, such as VCR clocks, may not have any mechanism to update their software.

Also, the change originated in the United States and is being followed in Canada, but not most other nations. That could befuddle conferencing systems and other applications that run in multiple countries at once.

In our hyper-networked age with data synchronizing on the fly — each year, it seems, there are fewer clocks that we have to manually change for daylight-saving time — it might be hard to imagine that computers' time could fall out of whack. After all, computers seamlessly keep their clocks in line by occasionally checking with "time servers" run by the government and other parties.

But what those time servers provide is "Universal Time" or Greenwich Mean Time. You tell your computer where in the world it is, and it performs the requisite adjustment to Universal Time. PCs on Eastern Standard Time now are subtracting five hours from Universal Time, but in daylight-saving time they will subtract four.

A common fix is a "patch" that reprograms systems with the updated start and end dates for daylight-saving time. Some of these updates are targeted at specific systems, while others have wider implications — such as one from Sun Microsystems Inc. for older versions of the Java Runtime Environment, which often fuels applications on computers and Web pages.

Microsoft planned to send its daylight-saving patch to Windows PCs with the "automatic update" feature Tuesday. Users with automatic updates turned off should download the patch from Microsoft. (New machines running Windows Vista are immune, since Vista was finalized after the 2005 law passed.)

However, computers running anything older than the most recent version of Windows XP, known as Service Pack 2, no longer get this level of tech support. Owners of those PCs should go into the control panel and unclick the setting that tells the machine to automatically change the clock for daylight-saving time. They have to make the change themselves when the moment arrives. (This is a sizable population; according to Gartner, Windows 2000 alone was still running 14 percent of PCs worldwide last year.)

For people who store their appointments in Microsoft Outlook or other desktop-based calendar programs — rather than dynamic, Web-based programs such as Google Calendar — the situation gets trickier. Patches for calendar programs are available, but appointments entered before a patch was applied might still be registered in standard time rather than daylight time — off by an hour.

Microsoft advises heavy calendar users to go online and download a small program known as "tzmove" — Time Zone Move — that can retrofit all previously booked appointments to the new daylight-saving rules. Other vendors offer similar tools for their systems.

Of course, it's likely not everyone would take that step, said Rich Kaplan, a Microsoft customer service vice president who oversaw the company's Y2K efforts and heads daylight-saving preparations. Hence Microsoft's advice to be cautious about meetings between March 11 and April 1.

"Because if one person applied the update, and one person didn't," he said, "you could end up there at the wrong time."